Archive for the 'Experiments' Category

Physicists have improved the measurement of the antiproton magnetic dipole moment, further narrowing how close to identical it is in magnitude (with opposite sign) to the proton value. I ran across a number of poor articles describing this experiment, all obviously cribbing from the same press release. One mentioned magnetic charge, another claimed that that the experiment determined the charge of an antiproton and seemed to confuse the Penning trap and the proton beam at the LHC. One put the emphasis on the act of trapping antiprotons, as if this had not been done before (it has).

This is a good one. There’s also a link to the free PRL.

[The ATRAP collaboration] look for a difference in the magnetic moments of the proton and antiproton. To enable this test, they precisely measure the magnetic moment of a single, trapped antiproton, achieving the most sensitive measurement to date of this quantity. They compare their result to the known value of the proton’s magnetic moment and find that the magnitudes are equal within experimental uncertainty, as predicted by the CPT theorem. Though there have been other tests of CPT with better precision overall, the work reported by ATRAP improves the limits on CPT violation in the difference of the proton and antiproton magnetic moments by nearly three orders of magnitude

I’ve been growing salt crystals. Unlike a pet, salt crystals won’t tear up the furniture, do their business on the carpet or need to be walked in the rain. Really I was testing to see if I could do this as a time-lapse project and wanted to test how long it would take. And it has fulfilled Hofstadter’s law: it always takes longer than you expect, even when you take into account Hofstadter’s law.

I took the standard approach of heating some water and dissolving a bunch of (uniodized) salt in it, letting it cool and pouring it into a beaker. And I waited. And waited. Finally, after a few weeks:

One thing I should have anticipated is how long it takes. There was some salt left in the pot when I poured the solution into the beaker, so I thought the crystallizing would begin quickly, but it didn’t. Plus, the evaporation was slow. I knew that boiling point elevation and freezing point depression are colligative properties (they depend on the number of dissolved atoms) so I reasoned that evaporation rate should be as well. And it is — there is Raoult’s law

The vapour pressure of an ideal solution is dependent on the vapour pressure of each chemical component and the mole fraction of the component present in the solution.

The vapor pressure of salt is very low, so as its concentration rises the total vapor pressure of the solution drops, and so does the evaporation rate. Rather than evaporating fully, one would expect it to reach an equilibrium with the atmosphere which would depend on the humidity. In fact, if the salt concentration were high enough, one might expect it to dehumidify the air, which is precisely what some people do. Salt concentrations are used in dehumidifiers — you expose the solution to the air and let is “grab” some water, then heat it up (often solar, for a completely passive system) to let the excess water evaporate, and cool it again in a cycle. Or you can have a solution with some solute left in the container, and as you “grab” the water, you dissolve more of the salt, so it can continue doing its job as long as there is more salt that can dissolve.

Another unexpected event in all of this is that I was getting salt crystallizing on the surface of the water. A small “raft” would float there until it grew massive enough that it would sink (or someone poked it). I had thought the crystallization would just build on any crystal that started up, but there are lots of small cubes rather than just a few large ones. Still, the biggest cubes are perhaps 10-20x larger on a side than the original grains.

My investigation asked the question of whether there is a secret formula in tree design and whether the purpose of the spiral pattern is to collect sunlight better. After doing research, I put together test tools, experiments and design models to investigate how trees collect sunlight. At the end of my research project, I put the pieces of this natural puzzle together, and I discovered the answer. But the best part was that I discovered a new way to increase the efficiency of solar panels at collecting sunlight!
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The tree design takes up less room than flat-panel arrays and works in spots that don’t have a full southern view. It collects more sunlight in winter. Shade and bad weather like snow don’t hurt it because the panels are not flat. It even looks nicer because it looks like a tree. A design like this may work better in urban areas where space and direct sunlight can be hard to find.

Update: I missed that he was measuring the open-circuit voltage output, not current, for his arrays.

Back when I was playing with a strong magnet dropping through a coil of wire I wondered how much energy I could extract from the dropped magnet and if I could do anything with it. The coil I was using was at least 15 cm in diameter, which means that I wasn’t capturing all of the flux lines from the magnet — the field of a dipole drops off as 1/r^3, so a smaller diameter would be much better and the slowing of the magnet could be noticeable, as we’ve seen before with someone dropping a magnet down a copper tube.

Since I’m a physicist, I wanted to quantify this. I didn’t have a copper tube handy, but I do have a roll of aluminum foil which is on a roll with an inner diameter of about 3.8 cm (1.5 in), which is a reasonably tight fit for my strong magnet. I set up my slow-motion camera and my ipod in stopwatch mode to double-check the timing (yes, it was shooting at a rate of 210 frames per second)

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I exported the video to individual frames to make it easier to analyze, and counted frames. The free drop takes about 0.25 seconds, give or take (it’s hard to tell exactly which frame represents release) and I estimate the distance as being about 32 cm (a foot-long roll = 30 cm, with the start just above and stop just below). The drop through the aluminum foil roll takes about 0.38 seconds. The freefall drop is easy to analyze: v = gt, and to double-check for g, just rearrange the familiar kinematics equation and solve. The drop time implies a speed of about 2.45 m/s at the exit. For g we get 10.2 m/s^2, so my little experiment seems good to 10% or better.

For the drop through the tube, we don’t know exactly what’s going on. There’s a damping force that varies with speed and eventually we would expect the magnet to reach terminal velocity. To get an estimate, though, let’s first assume it’s a uniformly lower acceleration. That would give us a value of 4.4 m/s^2 for the acceleration and an exit speed of 1.67 m/s. If we assume it hits terminal speed immediately then the speed would be 0.84 m/s. The truth is somewhere in the middle. There are probably several ways I could test this further, but the ones I can think of either require dropping the magnet from a distance above the tube, and it’s a tight fit, so it probably means lots of trials before I got lucky and got the magnet to drop in, or using a longer tube. I know aluminum foil comes in different lengths, but I only have the one. Since I want an idea of the energy extracted, let’s use the worst case value of 1.67 m/s.

I found the mass of the magnet using a small electronic scale and a plastic cup to keep the magnet away from the metal pan (where it might also be attracted to the interior or the case and mess up the measurement) and subtracted the mass of the cup. 60 grams.

Which means the magnet lost about 0.1 Joules of kinetic energy in the foil, in less than 0.38 seconds, or an average power of just over a quarter of a Watt, in that worst-case scenario. The best-case is 50% higher. And this is using aluminum — copper will give is a better result. Recall that Faraday’s law is
\(V = -frac{dphi}{dt}\)
Copper’s resistivity is about 2/3 of aluminum’s, so a given potential will drive about 50% more current and boost the resistive force owing to the larger field from the additional current. In other words, we can expect copper to be more efficient at converting the mechanical energy to electrical. It will more closely approximate the terminal-speed-quickly scenario, and it should have a smaller terminal speed.

What I want to do in the near future is wind a coil on one of these cardboard tubes and see if I can light up a little light bulb.

There was nothing left to do but award the prize to Fresnel. Poisson had put forward a consequence of light as a wave that was so ridiculous, so unlikely, that it couldn’t be explained by anything else. Fresnel was smart enough to come up with the theory. Poisson was smart enough to have proved Fresnel right, and proved himself wrong. Even though Dominique Arago had actually done the test, the tiny dot of light at the center of the shadow of a spherical object has ever after been called Poisson’s Spot. There is no perpetual motion in physics, but there is perpetual taunting.

If you want a short story about the essence of science, here it is. You have a model, it makes a testable prediction which will either confirm or falsify it. You do the experiment, find out that the model was right, and then tweak a detractor’s nose in perpetuity.

Budker uses atoms of the rare Earth element ytterbium to observe the largest extent of parity violation ever seen in atoms, larger by a factor of 100 compared to previous tests. His goal is to improve the precision of this measurement so that researchers could begin to use the parity-violating process to help measure the distribution of neutrons in nuclei.

Previous experiments from a dozen years ago used Cesium. The parity nonconservation was probed by looking at transitions between S orbital states (6S and 7S for Cesium). The electromagnetic transition between these states is highly forbidden (both having the same angular momentum and even parity) But because the orbitals are spherical they include the nucleus, and the weak interaction mixes in a tiny bit of a P-state transition (odd parity), which is allowed. It’s a tough experiment because you are looking for the small difference between the transitions when you reverse everything, and you start out with a small transition probability. I recall this because we tried trapping Francium when I was at TRIUMF, with the goal of providing an atom with a potentially larger parity-nonconserving subject. The effect varies with Z, and Francium was expected to give an effect that was ~18 times larger than Cesium. We weren’t successful (then) at trapping any, though we did succeed at piquing the interest of a nuclear watchdog organization.

If you ever want to get your head around the riddle that is quantum mechanics, look no further than the double-slit experiment. This shows, with perfect simplicity, how just watching a wave or a particle can change its behaviour. The idea is so unpalatable to physicists that they have spent decades trying to find new ways to test it. The latest such attempt, by physicists in Europe and Canada, used a three-slit version — but quantum mechanics won out again.

Flummoxed? Unpalatable?

It looks like a neat experiment. Maybe I’m missing something, but I don’t understand how the experimenters were flummoxed (the results agree with QM), nor do I see what they were getting at with the mention of relativity. And I don’t think I’ve found which-path behavior to be unpalatable, but then, I always used chocolate-covered gratings.

The roadway across the Golden Gate Bridge rises and falls as much as 16 feet depending on the temperature. When the sun hits the bridge, the metal expands and the bridge cables stretch. As the fog rolls in, the cables contract and the bridge goes up.

There’s also a 2-hour delay because of the thermal mass of the bridge.

It’s an ingenious plan with two major problems: first, the super-cold atom clouds are extraordinarily hard to make. Second, the best way to test gravity is to make sure that no other forces are acting on an experiment. Short of launching it into orbit, the best way to do that is to drop the whole experimental apparatus so that it goes into free fall.

Incredibly, Rasel and his team have now licked both problems. They devised a special self-contained canister that can automatically generate a BEC. They then dropped the canister from the 146-metre-high drop tower at the Center of Applied Space Technology and Microgravity in Bremen, Germany.

This is one of the experiments I mentioned in a discussion of chip-scale atomic physics a while back. And while it’s hard, I don’t know that it’s extraordinarily hard — the workshop I summarized was part of a push to move this type of technology forward. You get a lot of smart people thinking about the problem, trying different things, and you find solutions. But it is hard. It’s supposed to be hard. The hard is what makes it great.

Lead is, in principle, a shield against radiation, but freshly mined lead is itself slightly radioactive because it contains an unstable isotope, lead-210. “We could never use it for our experiment, which is exactly about keeping background radioactivity to a minimum,” says Ettore Fiorini, a physicist at the University of Milan-Bicocca and coordinator of the CUORE experiment. After it is extracted from the ground, however, lead-210 decays into more stable isotopes, with the concentration of the radioactive isotope halving every 22 years. The lead in the Roman ingots has now lost almost all traces of its radioactivity.